1. Field of the Invention
The present invention relates to a liquid ejection device and an image forming apparatus comprising a liquid ejection device, and more particularly, to a liquid ejection device which includes a suctioning mechanism for restoring ejection when a nozzle blockage or the like has occurred and which ejects ink onto a recording medium, or the like, to form an image, and to an image forming apparatus comprising such a liquid ejection device.
2. Description of the Related Art
As an image forming apparatus in the related art, an inkjet printer (inkjet recording apparatus) is known, which includes an inkjet head (liquid ejection head) having a plurality of liquid ejection nozzles arranged and which records an image on a recording medium by ejecting ink (liquid) from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
The inkjet head of an inkjet printer of this kind has pressure generating units. Each pressure generating unit includes: for example, a pressure chamber to which ink is supplied from an ink tank via an ink supply channel; a piezoelectric element which is driven by an electrical signal in accordance with image data; a diaphragm which constitutes a portion of the pressure chamber and deforms in accordance with the driving of the piezoelectric element; and a nozzle which is connected to the pressure chamber. The ink inside the pressure chamber is ejected in the form of a droplet from the nozzle when the volume of the pressure chamber is reduced by the deformation of the diaphragm. In an inkjet printer, one image is formed on the recording medium by combining dots formed by ink ejected from the nozzles of the pressure generating units.
An inkjet printer of this kind records an image by ejecting ink directly from very fine nozzles, and hence there are possibilities of ejection defects caused by the abnormality state of the nozzles and printing defects caused by ejection failures. Therefore, it is necessary to maintain the ink in a state which allows normal ejection at all times.
In particular, in an on-demand inkjet printer which ejects ink only when an image signal is input, there may be nozzles which do not perform ink ejection over a long period of time. In this case, the ink solvent may evaporate from the nozzles, the viscosity of the ink may increase, the ink may dry, and ejection defects may occur. Furthermore, phenomena of this kind may also occur if a recording operation is not carried out for a long period of time.
Moreover, in recent years, there has been a tendency for the number of nozzles to increase in inkjet head in order to increase printing speed. In order to restore a head suffering ejection defects, generally, a method is used in which the ink inside the pressure chambers is exchanged by suctioning the liquid inside the nozzles in the liquid ejection direction.
When suctioning of this kind is carried out in a head having a large number of nozzles, the ink inside all of the pressure chambers is exchanged. Hence, even the ink inside nozzles which perform normal ejections is also exchanged, and ink wastage thus occurs.
Consequently, in order to eliminate wasted consumption of ink, an “individual suctioning” technique has been contrived in which the nozzles of the inkjet head are divided into a plurality of regions, and suctioning is carried out only in a region including a nozzle which needs to be suctioned. When suctioning is carried out only for nozzles which require suctioning, from among the plurality of nozzles in the inkjet head, the ink is drawn in the liquid ejection direction from the nozzles where suctioning is carried out. Generally, the ink is supplied to the nozzles through a common flow channel, and the nozzles are interconnected via this common flow channel. Therefore, the force which moves the ink inside the suctioned nozzles in the liquid ejection direction, is also transmitted to other nozzles (non-suctioned nozzles) which are not suctioned, via the common flow channel. Due to this force being transmitted to other nozzles which are not suctioned, the ink inside these nozzles is caused to flow back. In this case, there is a possibility that air, or the like, may flow into the nozzles which are not suctioned.
In view of this, Japanese Patent Application Publication No. 11-314376 discloses a composition in which an opening and closing valve, or the like, is provided between each nozzle and the common flow channel inside the inkjet head. By adopting a composition of this kind, it is possible to prevent the ink from flowing back, and therefore it is possible to prevent air, or the like, from flowing into nozzles which are not suctioned.
However, if such an opening and closing valve is provided, then the cost of the inkjet head rises accordingly. In practice, it is difficult in manufacturing terms to provide such opening and closing valves inside an inkjet head, and the provision of the opening and closing valves leads to an increase in the size of the apparatus. In a large-scale head having a nozzle number of 10,000, if an opening and closing valve is provided for each nozzle, then there is a concern that production yield may decline. Hence, from the viewpoints of cost, manufacturing and production yield, it is not necessarily desirable to provide an opening and closing valve for each nozzle.
It is also possible to improve the invention described in Japanese Patent Application Publication No. 11-314376, by grouping together a certain number of nozzles (for example, 1000 nozzles) as one unit, and providing an opening and closing valve for each unit. Thereby, it is possible to reduce the number of opening and closing valves, and to remedy the aforementioned problems to a certain degree, but pressure variation occurs between units and this pressure variation causes adverse effects on image quality and hence this system is not practical.